That’d be a pretty small screen to be looking at while bouncing around on the lake, but if you find one make sure you use a back-up vice grip to hold it in place.
JMHO
Would also recommend Ram Mounts. I actually posted on here about their customer service, it’s terrific. They have a feature on their website that literally asks what device of i-phone, is it in a case or not, where you want to mount it, and it’ll give you the exact parts you need. I think I got my order in 2 days when I ordered a mount for a sled and helix.
Your pocket is going to be the best holder for your iPhone. They don’t deal with the cold. But if you really want to have it on the handlebars of your atv, Quad Lock is the best. You’ll need to have a power source for sure.